Misc:

• Timestamp.replace should support fold kwarg. ATM the kwarg exists but is ignored.
• Timestamp constructor should accept nanosecond kwarg (ENH: Timestamp constructor does not accept nanosecond arg #18898) (closed by ENH: Allow Timestamp to accept Nanosecond argument #19889)

Refactoring:

• move _nat_strings and _checknull_with_nat to a single canonical location
• put OutOfBoundsDatetimeError in a dependency-free util-like module. (Implement npy_dtime.pyx #17805)
• Possible redundancy between parse_datetime_string and parse_time_string update not redundant.
• De-privatize names in tslibs.parsing
• cython version of compat for string_types, StringIO
• Don't import DateParseError in tslib
• de-privatize nattype._checknull_with_nat (closed by Cross off a few tslibs-TODOs #18443)

Check if these need to be exposed or (in some cases) need to exist:

• _Period._comparables (closed by Cross off a few tslibs-TODOs #18443)
• Timedelta._validate_ops_compat
• Timedelta._binary_op_method_timedeltalike
• Timedelta._op_unary_method
• Timedelta._roundupdate Won't fix/correct-as-is
• Timestamp._roundupdate Won't fix/correct-as-is
• _Timestamp._get_fieldupdate Won't fix/correct-as-is
• _Timestamp._get_named_field
• _Timestamp._get_start_end_fieldupdate Won't fix/correct-as-is
• lib.arrmap (closed by Cross off a few tslibs-TODOs #18443)
• _libs.src.numpy_helper.infer_type

TZ Conversion:

• tslib._localize_tso appears to make a handful of unnecessary calls, see localize_tso unnecessary calls? #17944
• Is it the case that tslib._localize_tso argument obj will always have obj.tzinfo == None? This is the case in all existing usages.
• Inconsistency between tz_convert and tz_convert_single Inconsistency in tz_convert_single? #17734

Offsets:

• normalize attr should be relevant for __eq__ BUG: offsets normalize and __eq__ #17689 (closed by fix DateOffset eq to depend on normalize attr #21404)
• update docstrings
• Track down meaning behind comment # I suspect this is wrong for *all* of them. in BQuarterBegin.

Timestamp Methods (moved from #17876)-

• Timestamp classmethods should return cls, not hard-coded Timestamp
• ... if and only if is_timestamp is updated to recognized subclasses.
• NaT / timedelta should behave like NaT / Timedelta nat division by timedelta #17955
• _assert_tzawareness_compat doesn't actually return the declared return typeupdate but declaring it as void leads to build-time error because that mis-matches the except? -1
• Add timestamp method+test; closes #17329 #17906 implements Timestamp.timestamp as round(self.value / 1e9, 6) for consistency with datetime method. Are we sure we want to drop nanosecond information? Might it be better to update utcfromtimestamp for round-trip internal consistency?
• Timestamp.replace should handle freq Timestamp.replace should take freq kwarg #18544
• Timestamp.replace should handle DST gracefully Timestamp.replace should handle DST transition gracefully #18319 (closed by handle DST appropriately in Timestamp.replace #18618)

Follow-Up Cleanup:

• Fix camelCase naming in offsets
• docstrings for Timestamp.utcnow, utcfromtimestamp, fromtimestamp, combine
• ci/lint.sh stricter with pyx files
• simplify/correct NaT docstrings
• de-privatize _get_rule_month
• Possibly remove unique_deltas, asv etc Tslibs resolution #18034 (comment) update unique_deltas does appear to be more performant than the alternatives.
• add types, docstrings, de-privatize in some cases in tslibs.frequencies Move frequencies functions to cython #17746 (comment), Move frequencies functions to cython #17746 (comment)
• docstring for tslibs.resolution.resolution
• consider renaming is_null_datetimelike --> checknull_datetimelike (implement libmissing; untangles _libs dependencies #18357 (comment)) (2021-12-07 is_null_datetimelike has been removed)
• check for unit already PANDAS_FR_ns in conversion.ensure_datetime64ns
• Remove #noqad unused imports

Decision Making

• Have NaT not subclass datetime?
• Wrap datetime.date with Timestamp? Or possible treat it as Period(..., freq="D")? COMPAT: datetime.date comparisons with datetime64[ns] #17965
• Make Resolution singleton or use instance or [...]. See Tslibs resolution #18034.
• Consider making Timestamp.replace raise instead of returning an imaginary time (handle DST appropriately in Timestamp.replace #18618 (comment))

Issues:

• check if BUG: to_timedelta overflows without raising in some very particular cases #17037 is handled by BUG: overflow on Timedelta construction & arithmetic now raises #17640 Update It is not.

Questions:

• Shouldn't pd.Period('2017').day be None instead of 31? high-freq properties of low-freq Periods #18378

Unsorted

• Build warnings for timedeltastruct implement libmissing; untangles _libs dependencies #18357 (comment) (closed by Cross off a few tslibs-TODOs #18443)
• Ween off of dateutil internals, specifically import of _timelex in tslibs.parsing.
• Period tests for __sub__ with datetimelike check for datetime+period addition #18524 (comment)
• Fix missing tz arg in WIP: Pass missing tz arg in _parsed_string_to_bounds #18471.
• Make sure we aren't comparing tzinfo1 == tzinfo2 as that may be misleading for pytz
• investigate why an "extra layer" is needed in np_datetime.cstring_to_dts
• docstrings for np_datetime._string_to_dts, cstring_to_dts
• de-privatize frequencies._MONTH_NUMBERS etc, mismatch _MONTHS vs MONTHS

Low Priority:

• Set seen_datetime consistently in array_to_datetime array_to_datetime set seen_datetime consistently #18554